Those familiar with the drilling of oil and gas wells will appreciate the fact that a well is not necessarily precisely vertical at most points along the depth of the well. In like fashion, they appreciate the further fact that the earth, while formed of multiple layers, nevertheless may be formed of numerous layers at different angles with respect to the surface. To be sure, many wells are drilled that are substantially vertical, i.e. they are within two or three degrees of vertical as determined by a plumb bob or, stated in more elegant language, with respect to the gravity vector. There are many wells which are drilled where the well borehole may pass through formations of interest which are relatively well known and for which additional data is not required to enable well completion. Just as this is possible at one extreme, the opposite extreme of well position relative to the formations might occur. As an example, consider a highly deviated well which is drilled from an offshore platform where fifty or more wells extend from the platform, and very few of these can be described as simple vertical wells. Any number of the wells might deviate substantially from the vertical so that the well borehole has a portion which is perhaps vertical, another portion which is inclined at 60.degree. with respect to the vertical, another portion which is substantially vertical and so on. Indeed, there can even be a portion which is substantially horizontal to obtain large production flow rates from a relatively thin formation. Moreover, the well may intercept numerous formations of random thickness and having a variety of angular orientations. In that context, data obtained from the well must be oriented in space relative to the track or path of the well borehole and the formations penetrated thereby so that it is difficult to know what formations are implicated by the data. This is especially true for samples which are obtained from the formations.
There are three types of samples of interest to the present disclosure. The first is a core which is usually cut by a core bit while making the well borehole. That is, a cylindrical sample is cut by a cutting bit and then is removed with the bit by retrieving the hollow core storage device. This core is removed to advance the well depth. This will be described hereinafter as a conventional core sample or core. A second type of retrieved material is a cylindrical sample cut from and at right angles to the long axis of the conventional core. This sample will be called a core plug. The third type of retrieved material is a plug formed by a plug forming apparatus which is the device often known as a rotary sidewall coring tool which provides the much smaller plug sample. This particular device operates at right angles to the well borehole axis to remove a sample from the sidewall. Normally, this sample (a plug) is smaller in diameter and length than the core. It will described hereinafter as a plug. Where no distinction is required, both will jointly be described as a formation sample or simply as a sample. Samples are important in determining how to complete a well. A sample may be essential to determine whether or not a well should be plugged and abandoned or the further expense of completion should be incurred, that is, casing the well and forming perforations through the casing into the formations of interest. It is extremely important to know where the particular formation sample was obtained. Data of substantial importance can be obtained from the formations. In some instances, but certainly not all instances, the formations may be monolithic and devoid of information which is directional in nature. A more sophisticated description of earth formations may focus on this particular aspect. Similar to a tree which has a grain direction and inherent differences in characteristics with respect to direction, such differences in strength across or with the grain in like fashion point out formation directional characteristics. There may be directional differences in permeability, and it may desirable to also conduct directional petrographic studies. The formation sample many be subjected to directional examination under a scanning electron microscope. The formation may be formed of crystalline material which will then provide a different response with direction of irradiation when subjected to x-ray spectrometric studies. There may be a crystalline structure which defines a preferred fracture direction or stress direction. Because of these possible directional factors, it is important to know the position in space of a formation sample, and that includes both core and plug samples.
It has been difficult to obtain this information heretofore in highly deviated wells or in situations where the formations are suspected of departure from the rather simplistic model of parallel layers along the vertical borehole. By contrast, a relatively shallow well which is almost vertical relative to the gravity vector and which passes through a number of known formations (e.g., drilling a production well in a field that has been well explored) does not pose many problems. However, in a deviated well or a well near an upthrust of fracture in the region, the formation sample orientation is ideally determined by the mehtod and apparatus of the present disclosure so that the sample can be understood in context of its origination to thereby properly orient the sample data. This for instance enables determination of the imprinted paleomagnetic vector from formation samples. It enables determination of other data with respect to the location of the well borehole and the formations so that laboratory data obtained from formation samples can be meaningfully positioned in space. The present method and apparatus are directed to a system of determining the position of the tool comprising the cutting equipment which forms the formation sample. The tool is thus tracked in space and data regarding the position of the tool is processed so that the pathway of the well borehole can be determined. This data tracks or follows the locus of the borehole. The formation sample cutting device is thus positioned in space relative to the formations, and the core or plug is then obtained. This then enables the formation sample to be subsequentially retrieved to the surface so that the formation sample can be oriented and the test data obtained from the sample then is evaluated and the data can then be positioned in space to be much more helpful to well completion procedures.